A toner cartridge having a plug with a filter is described in U.S. Pat. No. 5,758,231 to Coffey et al. The plug has a labyrinth followed by a felt filter and having several entrance holes to the labyrinth used to vent the cartridge. Since this plug is in the side of the cartridge, the filtering will not be effective in a cartridge having toner in amount that reaches the plug.
Relocating the plug higher presents structural and operational conflicts. The use of a plug makes it necessary to have a hole in the toner hopper in which the plug is pressed. The size of the hole and the requirement to seal around it limits the locations on the hopper where the venting plug can be placed.